Allergies, which affect an estimated 50 million Americans, are the immune system's reaction to a normally harmless substance, such as pollens, certain foods, or medicines. There are two main types of allergies. Seasonal allergies occur at specific times of the year, while perennial allergies occur year-round. It is not clear why one person will develop allergies and another will not, although the tendency may be inherited. In many cases, allergies can be treated with medication, reducing contact with allergens, or shots.